I just finished a western today where I loaded cell lysates (each lane had the same cell line but with different plasmids in them) and I blotted using the same primary and secondary for all lanes. After development, I saw non-specic bands that were present in some lanes but not in others. Any idea to what is happening?!

Details please - how much protein or cells did you load per lane? What was the antibody against? Were the samples all collected at the same time? Were the samples all prepared in the same manner? Did you use (fresh) protease inhibitors?